is a good place to look for hard to find titles. It's a search engine that searches many book sales websites. They are linked to the main places like amazon and abebooks, biblio, alibris, thrift books and more. I have purchased through there many times.
I just searched Fr Fahey and there were 90 matches. Many repeat titles. A company called Loreto Publications seems to be reprinting several of them.
P.S. Looking up Loretto Publications I found they sell a set. http://loretopubs.org/fr.-denis-fahey-complete-set.html
I promise I am not affiliated.
vialibri led me there.